The present invention relates to apparatuses and methods for controlling brightness of light emitting diodes (LEDs).
It is often desired to have a dimming function in supplying power for lighting apparatuses. One commonly used dimmer is the well-known triac dimmer. A triac dimmer reduces its load power by chopping the load voltage that drives the current to the load during each half-cycle.
FIG. 1 shows a typical connection of a triac dimmer 14 and a LED light source 10. One end of the LED light source 10 is connected to the Neutral line N of the AC power source 12. The other end of the LED light source 10 is connected to a triac dimmer 14, which is in turn connected to the Live line L of the AC power source 12, through an optional switch 16. Most common dimmers are leading edge type dimmers in which the conduction at each half cycle is delayed from the zero crossing point so that the first voltage presented to the load after the zero crossing is the leading edge of the voltage.
The upper waveform of FIG. 2 shows the output voltage VTD waveform of a leading edge type triac dimmer 14 at a maximum brightness setting. The firing angle, which is the phase angle of the voltage at which the triac dimmer conducts, is at its minimum. The lower waveform of FIG. 2 shows the input current waveform ILS of the LED light source 10 when the triac dimmer 14 is at a maximum.
As the dimmer adjusts, the firing angle is changed. FIG. 3 shows the voltage VTD and current ILS waveforms when the firing angle is set to 90 degrees (about half brightness) and FIG. 4 shows the voltage VTD and current ILS waveforms when the firing angle is set to maximum firing angles (minimum brightness).
However, most dimmers are not able to power on if they are preset to maximum firing angle. The user has to turn the dimmer up for proper power on and then dim down to the desired brightness. Moreover at maximum firing angle, even if the dimmer is able to turn itself on, the dimmer output voltage would be too low to kick-start the LED light source circuitry.
There is a need for a power supply method and apparatus that does not suffer from these deficiencies.